1. Field of the Invention
The present invention relates to a highly flame-retardant, impact-resistant resinous composition having an excellent mechanical strength and a good moldability.
More specifically, the present invention relates to a highly flame-retardant, impact-resistant resinous composition excellent in the impact resistance, thermal stability, moldability and gloss, which consists essentially of (A) a graft copolymer obtained by bulk-polymerizing and suspension-polymerizing an aromatic monoalkenyl monomer and a vinylcyano monomer and/or a monomeric alkyl ester of acrylic acid or methacrylic acid in the presence of a diene type rubber component, optionally (B) a graft copolymer obtained by emulsion-polymerizing a monomeric mixture of the above-mentioned monomers in the presence of a diene type rubber latex, (C) a chlorinated polyethylene having a degree of chlorination of 25 to 45% by weight, (D) tetrabromobisphenol A or a derivative thereof and (E) antimony trioxide.
2. Description of Prior Arts
Recently, application fields of plastic materials have been broadened, and ABS resins are used as automobile parts, electric equipments, construction materials and other various molded articles in various fields because of their excellent impact resistance and moldability. However, with such broadening of application fields, various severe legal regulations are imposed on these plastic materials, and flame-retardant materials are now required to have not only a self-extinguishing property but also such a property that "flaming drippings" is not caused to occur at the time of combustion.
As means for rendering combustible ABS resins flame-retardant, there has generally been adopted a method in which at least one member selected from relatively low-molecular-weight organic flame retardants containing halogens in large quantities, halogen-containing polymers such as polyvinyl chloride, and inorganic compounds such as antimony trioxide is incorporated in an ABS resin. It is known that in order to impart to ABS resins such high flame retardancy that "flaming drippings" is not caused at the time of combustion, it is effective to use a halogen-containing organic compound, especially a halogen-containing polymer such as polyvinyl chloride, chlorinated polyethylene or the like, and antimony trioxide in combination.
As the typical method for production of ABS resins, there are known bulk-suspension polymerization and emulsion polymerization methods. For example, when an ABS resin is prepared according to the emulsion polymerization method, since a mixture of vinyl monomers is grafted to a rubber latex, the rubber content in the resin can optionally be changed. Further, since the particle size of the rubber latex is sufficiently small, a molded article of an ABS resin prepared by the emulsion polymerization method has an excellent gloss. However, an emulsifier or coagulant used in the preparation method is left in the resulting resin and has bad influences on the mechanical properties and thermal stability of the resin. Especially in the case a halogen-containing organic flame retardant and antimony trioxide are incorporated in such ABS resin in order to render it flame-retardent, if impurities such as the emulsifier and coagulant are left in the ABS resin, they promote combustibility of the ABS resin and excessive amounts of halogen and antimony components should be incorporated. Moreover, these impurities promote decomposition of the halogen-containing flame retardant. As a result, there are caused various troubles such as discoloration of the resin at the molding step and corrosion of an injection molding machine or a mold.
An ABS resin prepared by the bulk-suspension polymerization method is free of the above-mentioned impurities such as the emulsifier. Accordingly, this ABS resin has a good thermal stability, and when a flame retardant is incorporated in this ABS resin, the amount of the halogen or antimony component necessary for rendering the ABS resin flame-retardant can be reduced more than the case of the ABS resin obtained by the emulsion polymerization method. Further, the discoloration at the molding step can be remarkably reduced.
As another rubber-modified impact-resistant resin, there can be mentioned high-impact polystyrene. It is known that a larger amount of a flame retardant should be incorporated into an ABS resin than in this high-impact polystyrene if it is intended to attain the same level of the flame retardancy. Accordingly, high techniques are required for rendering ABS resins flame-retardant without other desired characteristics of ABS resins, namely higher impact resistance, rigidity and chemical resistance than those of the high-impact polystyrene. Chlorinated polyethylene which is often used as a flame retardancy-imparting component is poor in the compatibility with high-impact polystyrene, and therefore, even if chlorinated polyethylene is incorporated in high-impact polystyrene, the improvement of the impact resistance cannot be expected and such undesirable phenomena as reduction of the rigidity and laminar peeling are caused. Accordingly, from the practical viewpoint, the amount of chlorinated polyethylene incorporated into high-impact polystyrene should be limited to about 10% or less. In contrast, chlorinated polyethylene has a good compatibility with an ABS resin, and it is seen that when chlorinated polyethylene is incorporated into an ABS resin, the impact resistance is improved in proportion to the amount of chlorinated polyethylene incorporated. It is known that by incorporating a relatively large amount of chlorinated polyethylene into an ABS resin, it is made possible to render the ABS resin flame-retardant and compensate for reduction of the impact resistance caused by incorporation of antimony trioxide or the like. However, although the impact resistance is thus improved by incorporation of chlorinated polyethylene, the ridigity and moldability of the final resinous composition are reduced and the inherent characteristics of the ABS resin are considerably lost. In order to render an ABS resin highly flame-retardant while maintaining inherent characteristics of the ABS resin as much as possible, it is desirable to incorporate into the ABS resin chlorinated polyethylene, antimony trioxide and a halogen-containing organic flame retardant in an appropriate combination in minimum amounts.